Influence of transducer frequency on Doppler microemboli signals in an in vivo model

The purpose of this study was the comparison between 1 MHz and 2 MHz transducers in the detection of Doppler microembolic signals (MES). Intraoperative monitoring was performed over the arterial tubing of the extracorporal circulation circuit in 10 patients undergoing coronary artery bypass surgery, using a pulsed ultrasound machine (DWL X-4). 1 MHz and 2 MHz probes were mounted to insonate sequential tubing segments and all monitoring sessions saved on digital audio tapes (DAT). MES counts and intensity as detected by each probe were subsequently evaluated. Specific interobserver agreement was assessed using Cohen's kappa-statistic. Time delay in the appearance of MES between the transducers was minimal (0-0.1 msec). A total of 5293 (64%) MES were detected by both probes, 2796 (33.8%) only with the 1 MHz and 181 (2.2%) only with the 2 MHz probe. Significant differences in MES intensity were evident when comparing the 1 MHz to the 2 MHz transducer (16.4 +/- 0.1 and 11 +/- 0.1 dB with the 1 MHz and 2 MHz transducers respectively, p < 0.0001). Specific interobserver agreement was satisfactory (k = 0.78). Use of 1 MHz instead of 2 MHz transducers increases the yield of embolus detection in vitro. Evaluation of the applicability and performance of this transducer in clinical settings is warranted.